Do I have experience with these beings? No, but I can perhaps offer a bit of help when it comes to bonding with and interacting with them based on what I know of them and other members of the group of beings I personally would consider them to fall under in classification.

That is sidhe faie or at least sidhe like faie. Considering very few types of sidhe faie can be considered purely WA in nature this makes the Welsh Coblynau stand out among their Irish/Scottish cousins and even certain other Welsh faie.

All sidhe faie have a deep and close bond to nature, some are bonded to water others air and so on, this does not make them the same as elementals(who usually are either connected to a specific place, or show themselves as made of the element they are of, unidines and salamanders for example) but it does mean that since the Coblynau make their home in mines/caverns that become mines, their elemental affinity is soil/earth. They probably do have a strong connection to nature in general as well (like most sidhe) but are most strongly connected to soil/earth.

They made their home in mines when alive this explains why they are quiet in nature, loud noises in caverns and mines can easily lead to cave ins and death. Miners actually have to be extremely careful not to make to much noise for fear of calapsing the tunnels they work in.

They enjoy the soil and deep earth so it makes sense that they adore gems and precious metals, which come from within the earth, and considering they live in places rich in such things it makes sense they would lead humans to areas in caverns that are rich in such materials perhaps to share out of kindness.

The color connected to them is redish brown which is the color of soil rich in certain minerals and a color that is primarily associated with soil. They would enjoy anything that comes from the earth which is why raw gemstones and pine cones (pine cones are protective seed casing for pine trees and since pine trees take all their nuitrients directly from sun and soil it would count as being directly from earth/soil) are listed as treats. To bond with a being as connected to soil/earth as they are I would recommend being out in nature with them and anything you can do that is connected to soil (keeping house planets for example).

Just my in put, feel free to take it in whatever way you choose. The associations and treats I mentioned can also be found in the CH encyclopedia but I tried to explain ‘why’ some of those associations are associated with the coblynau, I hope that makes sense.